Implant extraction method and trephine drill bit for enabling the extraction

ABSTRACT

Method for extracting an implant installed in a bone, which comprises the application of a torque on an extraction tool in order to thread it in the interior of the implant until the implant-bone connection is broken, with the specific feature that the torque is applied in phases. In each phase a torque that is increasingly greater than in the previous phase is applied and the application of torque is then eased. This method brings about a gradual and controlled deformation of the head of the implant, with the result that the extraction tool is inserted more deeply into the implant. As a result, higher breakage torques may be applied without damage being caused to the extraction tool.

TECHNICAL FIELD

The invention relates to a method for extracting osseointegratedimplants, and a design for a trephine drill bit that is especiallysuitable for executing said method.

PRIOR ART

Bone implants in general and dental implants in particular arebiologically and mechanically designed to osseointegrate, in other wordsto join and fix themselves to the bone in a manner sufficiently strongfor them to be able to withstand the high mechanical stresses the bonein which they are installed has to withstand. Because of this, thebone-implant connection in practice is very strong and is designed notto break easily.

Nevertheless, implants sometimes have to be removed from the bone inwhich they are osseointegrated (in the event of their incorrectinstallation or a change in the medical treatment of the area, etc).Generally speaking, the removal of an implant is a complex task due tothe nature of the connection between the implant and the bone, which, asmentioned above, must be strong.

The removal of an osseointegrated implant is a process that can besomewhat traumatic for the bone and therefore for the patient. Thetraditional extraction procedure involves drilling with a hollow drillbit known as a ‘trephine drill bit’ around the implant. An areaincluding the implant itself and a section of the surrounding bone isdrilled. Once drilling is finished, the implant and said section ofsurrounding bone are removed. Unfortunately, this procedure leaves acavity in the patient's bone that is larger than the cavity originallyoccupied by the implant, which is a negative factor as said cavity isusually too large for fitting another implant; because of this, a boneregeneration process is usually necessary in order to fill the cavityprior to continuing with the implant treatment.

The applicant recently designed a new tool for extracting implants,described in patent application WO2009153372, which enables dentalimplants to be extracted without the need to drill the bone around theimplant. Said extraction tool comprises an external thread capable ofbeing threaded into the blind hole of the implant, in an oppositedirection to the external thread between the implant and the bone. Theextraction procedure is based on threading the extraction tool insidethe implant, by means of an additional torque application tool (e.g. thetool described in U.S. patent application Ser. No. 12/619079), slightlydeforming the head of the implant. The extraction tool is threaded untilthe rotation torque is so high that the fastening between the extractiontool and the implant reaches its maximum point and said rotation torqueis transferred to the implant-bone connection, eventually causing thebreakage of the connection and the unthreading of the implant (or thesimple breakage, in the event that the implant is not threaded to thebone). In order for this procedure to be executed, therefore, theextraction tool must be able to withstand high torques and to transfersaid torques to the implant. Then, by means of the aforementionedextraction tool, the implant is removed cleanly, leaving behind it onlythe bone cavity occupied by the implant. As a result, there is no needto perform a subsequent bone regeneration phase, in addition to the factthat the process of extracting the implant is evidently made lesstraumatic for the patient.

It is an objective of this invention to provide an improved implantextraction method, based on the application of a high extraction torqueon the implant, which is in turn transferred to the osseointegratedimplant-bone connection. The method should make maximum use of thetorque applied, with the result that it is not necessary to applyexcessively high torques to bring about the breakage of said connection.The objective is that the surgeon should not have to exert a largeamount of effort to break the implant-bone connection (as there is arisk of the extraction tool breaking under very high torques), and that,with the application of a specific maximum torque, stronger bone-implantconnections may be broken than with the aforementioned method.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of this invention to provide a method for extracting animplant installed in a bone, which comprises the application of torqueon an extraction tool in order to thread it in the interior of theimplant until the implant-bone connection breaks, with the specificfeature that in said method the extraction torque is applied insuccessive phases. In each phase, an extraction torque is applied on theextraction tool until a specific maximum torque value is reached; theapplication of said torque is then eased. The specific maximum torquevalue increases in each successive phase. In other words, in theinventive method an increasing extraction torque is applied on theextraction tool until a specific maximum torque value is reached; theapplication of torque is then eased or stopped; an increasing extractiontorque is applied once more up to a specific maximum torque valuegreater than the previous one; the application is eased once more, andso on until the connection between the implant and the bone is broken.

The method according to the invention, based on the gradual applicationof increasing torques in phases and with pauses, provides better resultsthan the basic method described in WO2009153372, based on theapplication of an increasing extraction torque on the implant until,directly and without any pauses, the connection between the implant-boneis broken. It has been shown in testing that this method brings about agradual and controlled deformation of the head of the implant, with theresult that the extraction tool becomes more deeply inserted into theimplant. The new effect is very important (in view of the fact that theextraction tool usually has a top part that increases in diameter)because the area of the extraction tool that withstands the stresses hasa larger diameter and is therefore stronger, thereby enabling theapplication of higher breakage torques without damage being caused tothe extraction tool.

DESCRIPTION OF THE DRAWINGS

Details of the invention can be seen in the accompanying non-limitingdrawings:

FIG. 1 shows a cross-sectional view of a dental implant and anextraction tool involved in the execution of the method according to theinvention.

FIG. 2 shows an embodiment of the trephine drill bit according to theinvention.

FIG. 3 shows a second embodiment of the trephine drill bit according tothe invention.

FIG. 4 shows an enlarged view of part of FIG. 3.

FIG. 5 shows a graph of the temperature the bone acquires with respectto the trephining time, the graph showing both the maximum temperaturevalues generated using the trephine drill bit according to the inventionand the maximum temperature values generated using a conventionaltrephine drill bit.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a method for extracting an implant installed ina bone, the method comprising the application of a torque on anextraction tool in order to thread it in the interior of the implantuntil the implant-bone connection breaks, with the specific feature thatin said method the extraction torque is applied in successive phases. Ineach phase, an extraction torque is applied on the extraction tool untila specific maximum torque value is reached, and the application of saidtorque is then eased, said specific maximum torque value increasing ineach successive phase. In other words, according to the inventivemethod, the extraction tool is placed inside the head of the implantand, by means of a torque-application tool, an increasing and gradualextraction torque is applied in the fastening direction of theextraction tool (generally in an anti-clockwise direction) until a firstmaximum torque value is reached. Said torque application tool ispreferably an adjustable maximum torque wrench, such as the onedescribed in U.S. patent application Ser. No. 12/619079. When this pointis reached, the extraction tool is unthreaded, without it having to beremoved completely. After it has been loosened and the inherent stressesin the extraction tool have been released, a torque is applied once moreon the extraction tool until a second maximum torque value, greater thanthe previous one, is reached. This methodology is repeated, safeguardingthe useful life of the extraction tool until the osseointegrationbetween the implant and the bone is broken and the implant extracted.

Although the use of a multi-torque (adjustable) wrench or severalfixed-torque wrenches of different values is preferable for carrying outthe inventive method, the method may also be performed used a singlefixed-torque wrench. In this respect dental professionals should usetheir judgement, which will help them decide the correct point at whichthe application of torque should be eased in each phase.

The method described herein enables the controlled deformation of thehead of the implant as the extraction tool is threaded and,consequently, the deeper insertion of the extraction tool inside theimplant. As a result, and given that the diameter of the extraction toolincreases towards its apex, the area of the tool that applies the torqueon the implant and withstands the stresses has a larger diameter (and istherefore stronger). In consequence, the extraction tool is capable ofapplying and withstanding higher torques. The surgeon is thus able breakstronger implant-bone connections without breaking the extraction tool.In laboratory testing it has been shown that the inventive procedureallows torques up to 60% higher to be applied without the extractiontool breaking.

Preferably, the extraction method according to the invention comprisesthe additional step, in each phase (once the torque has been eased andbefore the application of a greater extraction torque in the nextphase), of applying a torque in a direction opposite to the extractiondirection in order to detach the extraction tool from the implant. Thisenables the extraction tool to enter further into the implant. As theextraction tool has a diameter that decreases towards its apex (in otherwords, a diameter that increases towards its top area), the fact that itis introduced further into the implant means that the active diameter ofthe extraction tool is increased. As a result, the strength of theconnection between the extraction tool and the implant is increasedconsiderably.

The inventive method optionally comprises the additional step of makingin incision with a trephine drill bit in an external part of theimplant-bone connection. The outermost area of the implant-boneconnection is the area in which Bone-Implant Contact (BIC) is at itsgreatest. In fact, it has been estimated that when the implant has beeninstalled in the bone for a considerable period of time, BIC in saidoutermost area may account for up to 80% of the total BIC betweenimplant and bone. Trephining in this outermost area may thus result in aconsiderable reduction in the maximum torque that must be applied on theextraction tool.

FIG. 1 shows a cross-sectional view of an implant (1), in this case adental implant, installed in a bone and to which an extraction tool (2)is connected. The extraction tool (2) is of the type that is used toperform the method according to the invention, thus comprising athreaded body (4) that is threaded into the blind threaded hole (3) ofthe implant (1). The threaded body (4) is inserted until it is sostrongly threaded that the application of even greater torques on theextraction tool (2) causes the implant-bone connection to break.

As shown in the figure, in this case extraction is made easier bytrephining on the outermost area (5) of the implant-bone connection. Thetrephining has two main effects. Firstly, it enables the point ofapplication of the greatest stresses on the extraction tool (2) to betransferred to the implant (1), from the original point (7), withouttrephining, to the current point (6), which corresponds with an area ofbone that is not as strong as that of the original point (7). Secondly,the stress on the current point (6), with trephining, is considerablygreater than the stress on the original point (7), without trephiningThis effect may be seen in FIG. 6, which shows the stress applied on thebone in accordance with the depth of the point of application. As can beseen, when trephining is carried out for example up to a current point(6) at a depth of approximately 4.2 mm, the stresses applied to the bonefrom this point (the dashed line) have maximum values of approximately200 MPa, whereas when trephining is not carried out the stress appliedon the bone (the dotted line) has much lower maximum values, of 150 MPa,at the zero depth point (the original point (7) of FIG. 1). In otherwords, slight trephining of the outermost area (5) of the implant-boneconnection causes a fitting effect that concentrates the stresses causedby the extraction tool (2) on the bone. The combination of both effectsbrings about a reduction in the maximum torque necessary to break theimplant-bone connection.

FIG. 2 shows an elevated view of an embodiment of the trephine drill bit(8) according to the invention, which enables the trephining describedabove to be carried out safely and accurately on the outermost area (5)of the implant-bone connection. The outermost area of the trephine drillbit (8) comprises a reduced area (9) of a smaller diameter than the restof the body of the trephine drill bit (8). Said reduced area (9) ends ina stop (10) that guides the surgeon during trephining and tells him/herwhen they have to insert the trephine drill bit (8). The stop (10)operates in such a way that, when the stop (10) comes into contact withthe bone while trephining, the stop (10) does not cut or pull the bonebut merely rubs against it. Because of the fact that the reduced area(9), i.e., the serrated cutting part on the end of the trephine drillbit (8), is especially thin and therefore requires a minimal amount offorce for trephining, the rubbing of the stop (10) is enough tocounteract said force. As a result, the rubbing of the stop (10) againstthe bone allows the surgeon to become aware that the trephine drill bit(8) has been inserted to the required limit

Preferably, the height (h) of the reduced area (9) is between 2 and 8 mmAn especially advantageous embodiment is that in which height is between4.5 and 5.5 mm, in which the depth value of the trephining depth isoptimised (in other words trephining is as brief but also as effectiveas possible in terms of the maximum torque reduction provided).

FIG. 3 shows a partial schematic cross-sectional view of a secondembodiment of the trephine drill bit according to the invention. Herethe trephine drill bit (8′) comprises a body (11) and a tip (12), thelatter being of a smaller diameter than the body (11) and beingconnected to it by means, for example, of a transverse fixing member(13), as shown in the figure. In this case, the reduced area (9) of thetrephine drill bit (8′) is on the tip (12) itself, while the end of thebody (11) provides the stop (10). The advantage of this embodiment isthat it allows a body (11) of one material (generally durable and notfragile) and a tip (12) of another material (generally hard) to be used.

FIG. 4 shows an enlarged view of the area of FIG. 3 where the stop (10)is situated, more clearly illustrating all the aforementioned elementsof the trephine drill bit (8′).

FIG. 5 shows a graph of the temperature the bone acquires in accordancewith the trephining time, the maximum temperature values generated usingthe trephine drill bit according to the invention (dashed line) and aconventional trephine drill bit (dotted line) being compared. As can beseen, with the trephine drill bit according to the invention, which isused for drilling for a shorter period of time, the maximum temperaturevalues acquired by the bone surrounding the trephine drill bit are lowerthroughout the trephining (lasting between 0 and 50 seconds in the caseshown) in comparison to the maximum temperature values generated whentrephining with a conventional trephine drill bit (a longer period oftrephining lasting between 0 and 160 seconds). As a result, the use ofthe trephine drill bit according to the invention offers the additionaladvantage of preserving the quality of the bone surrounding the implantduring the extraction.

The invention claimed is:
 1. A method for extracting an implant installed in a bone comprising: a) inserting a threaded extraction tool having a tapered threaded body into an implant cavity; b) causing the threaded extraction tool to rotate and thread into the implant cavity, the threaded extraction tool meeting an increasing resistance to rotation originated by an increasing friction between the threaded extraction tool and inner walls of the implant cavity, whereby the threaded extraction tool is caused to rotate by applying an increasing torque on said tool in order to overcome said increasing resistance to rotation; c) ceasing rotation of the threaded extraction tool when said torque applied on the threaded extraction tool to produce rotation of said tool has reached a specific maximum torque value; d) partially unthreading said threaded extraction tool relative to the implant; e) and without removing said threaded extraction tool from the implant, carrying out a plurality of repetitions of steps b) through d), wherein in each execution of step c) a greater specific maximum torque value is reached than in the previous execution of step c), until an osseointegrated connection between said implant and the bone breaks, thereby allowing the implant to be removed.
 2. The method according to claim 1, comprising the making of an incision with a trephine drill bit on an external part of the implant-bone connection in order to reduce the maximum torque that must be applied on the extraction tool. 